In the field of medicine the use of laser devices for treatment purposes is becoming commonplace. Such devices are used for cauterization of wounds, excision of tissue, selective thermal absorption in tissue, welding of tissue through the formation of scar tissue, and the like. Recent developments in intravascular treatment point to recanalization of atherosclerotically occluded vessels virtually anywhere in the human body, including the relatively small vessels which supply the heart muscle itself. Such developments are described in copending U.S. patent application Ser. No. 07/019,755, filed Feb. 27, 1987 by the present inventor and Dan Rink and Garrett Lee.
Generally speaking, lasers designed for medical use should be highly controllable with respect to the power output level of the laser, and the duration of the laser illumination. Paradoxically, although the laser output power rarely exceeds approximately 20-25 watts for medical treatment purposes, the amount of power used to generate this laser power level is extraordinarily high. In lasing medium which operates continuously, kilowatts of power are consumed, even on a standby basis, so that a few watts of light energy can be delivered briefly or sporadically to the desired application site. The heat generated in the lasing cavity and in the power supply require that an external cooling system be provided. Thus an external source of cold water is generally required, and hundreds of gallons of water are expended in relatively short procedures. External cooling systems add to the complexity and expense of a medical laser, and create further connection and maintenance problems.
In pulsed mode laser devices, there is the opportunity to save power consumption since the lasing medium is operated only sporadically. However, pulsed mediums do not react predictably when first activated, due to thermal and dimensional effects. For example, when a NdYAG laser rod is firt pumped by an arc lamp, the rod experiences a rapid thermal buildup which alters the axial dimension of the rod. As the rod changes in shape, the quality of the laser output pulse is severely affected. Thus prior art devices may provide erratic power outputs in pulse or burst modes of operation.
This problem is complicated by the fact that many prior art pulsed laser systems measure the power output of each pulse (by any of several techniques known in the art), compare that power level to a preselected level, and in response alter the intensity or period of succeeding pulses. The inherent time lag of this process, together with the averaging errors and the potential instability in such level-hunting systems, can create unacceptably erratic performance.
Prior art lasers have employed high voltage DC power supplies to pump and fire a pulsed mode laser, and pseudo-continuous operation may be added by charging capacitors with the high-voltage power and sequentially connecting the capacitors to a flash lamp or the like to fire the laser to produce a plurality of time-separated pulses. However, such power supplies are expensive and inefficient, and there is a limit to how many capacitors can be provided in a practical laser apparatus.
Another difficulty found in medical and other forms of work with lasers is that prudent safety considerations that all personnel wear laser safety goggles whenever they are in an area in which a laser is in use. Particularly in medical settings such as a surgical operating room, the surgeons and assisting staff, the anesthesiologists and the patient must be equipped with safety goggles. Often the goggles interfere with other equipment, such as sterile masks, the anesthesia mask, and the like, and are a distraction at best. There is no remedy for this problem known to the present inventors.